The present invention, in some embodiments thereof, relates to inkjet printing technologies, and more particularly, but not exclusively, to a process of inkjet printing on a continuous printing apparatus using a coagulating ink composition.
Digital printing is becoming the method of choice for many large-scale industrial manufacturers of colored design fabrics for the garment industry. This method allows a manufacturer to respond rapidly to the ever changing demands of the dynamic fashion market. Nonetheless, the use of digital printing is limited particularly when the demand of the fashion market grows in terms of a wider variety of fabrics (natural fiber, synthetic, stretch, etc.), a more demanding use for the product (wash fastness, “breathing”, stretchability, etc.), a wider color gamut on lightly-, darkly-colored and/or stretchable fabric, and higher resolution designs.
The type of fabric and the use thereof determine the suitable type of digital ink. The inks that have been used most broadly for high color gamut, high durability and high resolution printing on industrial scale, at least until the recent years, have been dye-based digital inks, which are typically classified into acid, reactive and dispersed inks. One of the disadvantages of reactive dye-based inks is that they require the use of a fabric with a specific pre-treatment and the multi-step fixation process required per each type of ink, wherein acid and reactive inks require moist heat as in steaming for fixation, and dispersed dye inks require dry heat fixation. The advantage of the dye-based inks is the wider-achieves color gamut.
Digital pigment-based inks that typically do not require any pre-treatment or post-treatment. These inks are typically based on a suspension of micro- and nano-sized colored particles, suspended in a resin-based emulsion. While pigment-based digital inks provides for a reasonable color gamut, the color gamut achieved with such inks is not as wide as achieved with dye-based inks. Yet, the advantage of pigment-based digital inks is the ability to print onto the widest range of fabrics including cotton, silk, rayon, cotton/poly blends, and some polyester in a variety of weights, while no pre-treatment is necessary, allowing the acquisition of fabrics from any source and market.
In the recent years, a family of pigment-based digital inks, suitable for printing on any type of fabric, dark or light colored, absorptive and stretchable, have been developed and commercialized (see, for example, U.S. Patent Application Nos. 20040252173, 20070103528, 20070103529, 20070104899, 20110032304, 20120040148 and 20140036014, U.S. Pat. No. 7,134,749 and International Patent Application Nos. WO/2005/115089, WO/2005/115761 and WO/2011/018786), each of which being incorporated by reference as if fully set forth herein. These ink compositions are based on a chemical and/or physical property of the ink components, which can be selected so as to coagulate or precipitate upon contact with another substance on the surface of the substrate. This coagulate or precipitate, or otherwise immobilization of the ink droplet, afford several major achievements—the ink is not absorbed by the substrate in case of textile and other absorptive materials, hence there is very minimal penetration and crossing-over of ink into the fabric and its back-side; the droplets preserve their original size and shape of microscopic droplets by not spreading over and/or into the substrate, thereby affording a sharper image and better color definition; and the ink forms a film over the substrate which can be selected so as to be stretchable and breathing, allowing the formation of a digital image on a stretchable substrate without sacrificing any of the color image quality even after it has been pulled and stretched. This family of ink compositions transformed the garment industry by allowing the manufacturer to use any non-pretreated fabric at any scale, including in “roll-to-roll” printing machines (continuous substrate printing machines), while not sacrificing the color gamut and image resolution which have been previously achievable only by dye-based digital inks.
In some digital and non-digital printing systems, such as in a “roll-to-roll” printing systems, it is required that the fabric does not slip or deform during printing process in order to prevent loss of print definition. To that end, the fabric is held stretched and in controlled motion by being temporarily affixed to a flat support, which is typically made of a flexible water-proofed rubber-like material. The surface of the support which faces the fabric is made to be removably adhesive so as to afford the temporary affixation of the fabric to the support; hence, the support is typically referred to collectively as an adhesive blanket, a sticky conveyor belt, or a sticky blanket. The fabric is therefore usually temporarily affixed to the adhesive blanket, or stuck onto it with a solvent- or water-based polymeric resin/gum or a thermoplastic polymeric adhesive (removable adhesive). After printing, the fabric is separated from adhesive blanket, and the belt is washed to remove any fiber and ink residues transferred through the cloth or beyond its edges.
Typically, the polymeric coat loses its stickiness (tackiness) due to loose fibers released from the fabric during the printing process that cover the surface of the adhesive blanket, particularly when used to convey long stretches of fabric. This problem is solved by cleaning the blanket continuously during the printing process by means of an elongated brush dipped in a water basin and positioned underneath conveyor belt. The wet brushing releases the fibers from the adhesive layer, thereby renewing the capacity of the belt to affix the fabric.
However, while this water-based cleaning process of the adhesive blanket is effecting in removing loose fibers, it not effective when using a pigment-based ink composition that has the capacity to coagulate on the surface of the fabric, as some of the ink does make its way to the adhesive surface through the fabric, thus coating the blanket adhesive polymeric coat with a film of ink that is not removed when brushed with water. When the adhesive blanket is coated with a film made of coagulated ink residues, oily residues and loose fibers from the fabric, rendering it non-sticky, the operator of the printing machine is forced to remove the spent adhesive layer, typically using costly and sometimes hazardous organic solvents, and reapply a new adhesive layer. The process of reapplying an adhesive layer on the conveyor belt take a toll of human resources and other resources, and creates a long hold in the production process (downtime) at a great cost to the printed fabric manufacturer.